In general, when the potential difference between an internal reference voltage of a DC-DC converter and a power supply voltage is not very large, a low-noise series regulator with a simple circuit configuration is often used. But, when an output circuit of a DC-DC converter carries out a switching operation, the current flowing to the output line of an internal reference voltage-generating circuit in the DC-DC converter changes considerably. To be able to respond to a steep current change, an output circuit system of the internal reference voltage-generating circuit may be designed as a source follower or emitter follower.
However, in a source follower or emitter follower, because there is a voltage drop of the source (emitter)-to-gate voltage Vgs or Vbe, the operating voltage range of the internal reference voltage-generating circuit is also reduced.
To secure a wide operating voltage range, it is effective to use a low-dropout regulator (LDO: Low Dropout Regulator) in which a large capacitance is added to the output line of the internal reference voltage-generating circuit. The reason why a large capacitance is added is that the frequency response characteristic is slow in the conventional LDO. The LDO is a series regulator that operates even if the potential difference between the input and output voltage is about 0.1 V.